In a typical photolithography machine of this kind, a light beam emitted from a light source enters a fly's eye lens. Then, a secondary light source constructed from many light source images is formed on the rear side focal plane thereof. A light beam from the secondary light source is limited through an aperture stop arranged in the vicinity of the rear side focal plane of the fly's eye lens, and then, enters a condenser lens. The aperture stop limits a shape or a size of the secondary light source to a desired shape or a desired size according to desired illumination conditions (exposure conditions).
A light beam condensed by the condenser lens illuminates in a state of superimposition a mask in which a given pattern is formed. Light passing through the pattern of the mask forms an image on a wafer through a projection optical system. Thereby, the mask pattern is projected and exposed (transferred) on the wafer. The pattern formed on the mask is highly integrated. Therefore, in order to correctly transfer this fine pattern on the wafer, it is essential to obtain a uniform illumination intensity distribution on the wafer.
In these years, a technique, wherein coherency σ of illumination (σ value=aperture stop diameter/pupil diameter of projection optical system, or σ value=numerical aperture on emission side of illumination optical system/numerical aperture on entrance side of projection optical system) is changed by changing a size of a secondary light source formed by a fly's eye lens by changing a size of an aperture part (light transmittance part) of an aperture stop arranged on an emission side of the fly's eye lens is noted. Further, a technique, wherein a depth of focus and a resolution of a projection optical system are improved by limiting a shape of a secondary light source formed by a fly's eye lens to an annular shape or a quadrupole shape by setting a shape of an aperture part of an aperture stop arranged on an emission side of the fly's eye lens to an annular shape or a four hole shape (that is, quadrupole shape) is noted.
As described above, in the related art, in order to perform modified illumination (annular illumination or quadrupole illumination) by limiting the shape of the secondary light source to the annular shape or the quadrupole shape, a light beam from a relatively large secondary light source formed by the fly's eye lens is limited by the aperture stop having the annular or quadrupole aperture part. In other words, in the annular illumination or the quadrupole illumination in the related art, a substantial part of the light beam from the secondary light source is blocked by the aperture stop, and does not contribute to illumination (exposure). In result, due to light amount loss in the aperture stop, there are disadvantages that an illumination intensity on the mask and the wafer is lowered, and throughput as a photolithography machine is lowered.
In order to solve these disadvantages, a technique, wherein a light beam from a light source is transformed into a light beam having a given cross section shape and an angle element by an optical integrator constructed from a diffractive optical element or a micro lens array (refractive optical element), and an illumination field in a desired shape (annular shape, quadrupole shape and the like) is formed on an entrance surface of a fly's eye lens by a diffractive optical element as a light beam transformation device based on this light beam (then, a secondary light source in a desired shape is formed on an emission surface of the fly's eye lens) is suggested.
However, in this related art, for example, when a pulse oscillation type laser light source such as a KrF excimer laser light source and an ArF excimer laser light source is used, a diffractive optical element or a micro lens array as a light beam divergence device is irradiated with pulse laser light having very high energy density. In result, a micro channel (minute hole) is generated on an emission surface of the diffractive optical element or the micro lens array which is formed by an amorphous (noncrystalline) material such as silica. Consequently, there is a disadvantage that a transmittance is lowered, which causes light amount loss.
In the view of the foregoing problems, it is an object of the invention to provide an illumination optical device, wherein a micro channel is not generated practically in a diffractive optical element or a refractive optical element arranged in an optical path of pulse laser light having high energy density. Further, it is another object of the invention to provide a photolithography machine and an exposure method capable of performing good projection exposure with high throughput by using an illumination optical device, wherein a micro channel is not generated practically in a diffractive optical element or a refractive optical element arranged in the optical path of the pulse laser light having high energy density.